Refactor storage-init command
Refactor storage-init command

Use internal API to get a reference to the storage backend and remove
the need to use stevedore.

Change-Id: Iade2b11e8a4b1fe5199b303c1ba537d89083154f

openstack/cloudkitty,stackforge/cloudkitty,openstack/cloudkitty,stackforge/cloudkitty